[kerx] Comment

src/hb-aat-layout-kerx-table.hh

@@ -317,7 +317,7 @@ struct kerx
         goto skip;
 
       if (table->coverage & KerxTable::CrossStream)
-        goto skip; /* We do NOT handle cross-stream kerning. */
+        goto skip; /* We do NOT handle cross-stream kerning.  None of Apple fonts use it. */
 
       reverse = bool (table->coverage & KerxTable::Backwards) !=
 		HB_DIRECTION_IS_BACKWARD (c->buffer->props.direction);

src/hb-ot-kern-table.hh

@@ -463,6 +463,7 @@ struct kern
 		       hb_buffer_t  *buffer,
 		       hb_mask_t kern_mask) const
     {
+      /* We only apply horizontal kerning in this table. */
       if (!HB_DIRECTION_IS_HORIZONTAL (buffer->props.direction))
         return;